Medical devices, such as cardiac pacemakers and implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include cardiac pacing pulses or cardioversion/defibrillation (CV/DF) shocks.
The medical device may sense cardiac electrical events attendant to the intrinsic heart activity for detecting an abnormal intrinsic heart rhythm. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation therapy may be delivered to restore or maintain a more normal rhythm of the heart. For example, an ICD may deliver pacing pulses to the heart of the patient upon detecting bradycardia or tachycardia or deliver CV/DF shocks to the heart upon detecting tachycardia or fibrillation.
The ICD may sense the cardiac electrical signals from a heart chamber and deliver electrical stimulation therapies to the heart chamber using endocardial electrodes carried by transvenous medical electrical leads. In other cases, a non-transvenous lead may be coupled to the ICD, in which case the ICD may sense cardiac electrical signals and deliver electrical stimulation therapy to the heart using extra-cardiovascular electrodes. The energy of a therapeutic electrical stimulation pulse required to effectively stimulate the heart using the extra-cardiovascular electrodes is typically greater than the energy required to stimulate the heart using endocardial electrodes. A pacing circuit may include a holding capacitor that is charged to a pacing voltage amplitude for generating a pacing pulse according to the pacing pulse energy required to capture the pacing heart using the pacing electrode vector that is available.